1. Field of the Invention
The present invention relates to an antenna device and, more particularly, to a surface-mount type antenna device suitable for use in, for example, a mobile communication system or a wireless LAN (Local Area Network) system.
2. Description of the Related Art
Referring to FIG. 14, a conventional surface-mount type antenna has a rectangular dielectric substrate member 31 made of ceramics or a resin and having a through bore 32. The wall surface defining the through bore 32 has a radiating electrode 32a formed therein. A capacitance loading electrode 35 formed on one side surface of the dielectric substrate member 31 is connected to the radiating electrode 32a. A feeder electrode 33 is provided on the side surface of the dielectric substrate member opposite to the capacitance loading electrode 35 and is connected to the radiating electrode 32a. Ground electrodes 34a and 34b are disposed at both sides of the feeder electrode 33. In use, this surface-mount type antenna is mounted on a printed circuit board, with the feeder electrode 33 connected to the input end of an RF circuit.
In order to miniaturize the conventional surface-mount type antenna, it is necessary to increase the dielectric constant of the dielectric substrate member 31 so as to enhance the capacitances between the capacitance loading electrode 35 and the ground electrodes 34a, 34b. An increase in the dielectric constant of the dielectric substrate member 31 causes an increase of the Q factor, with the result that the frequency band of the antenna is undesirably narrowed.
Considering that the frequency response of a communication apparatus may be shifted due to the influence of a housing, a nearby object, or the like, it is not preferred that the frequency band of the antenna is narrowed.